Detection of a load in the axial direction of a cylinder unit is required, for example, when a limit load causing a damage of the concerned cylinder unit is to be detected during the use of the unit for preventing such damage, when the weight of a load or material is to be detected, when change of stress applied by a load is to be traced, or when a safety means is to be operated at a predetermined load so that a component operated by the cylinder unit is not damaged. Such detection of load applied to a hydraulic cylinder unit in the axial direction of the same is generally made by detecting the pressure of fluid for operating the cylinder unit, as shown, for example, in U.S. Pat. No. 3,477,854 and U.S. Pat. No. 3,489,294. That is, the former U.S. patent discloses a hydraulic jack weighing device for aircraft and the like wherein a separate fluid chamber is provided which is communicated to a fluid chamber of a fluid cylinder for the jack through a fluid passage, a pressure plate of a deflectible material is arranged at the bottom of the former fluid chamber, and gauges for detecting the deflection of the pressure plate are secured to the bottom face of the pressure plate so that load applied to the fluid cylinder is detected through the fluid pressure and the deflection of the pressure plate whereby the weight of an aircraft or the like which corresponds to the load is measured. In the U.S. Pat. No. 3,489,294, a load limit control for hoisting apparatus is disclosed in which a signal corresponding to a load actually applied to the hoisting apparatus is compared with a safety limit signal corresponding to a safety limit load so that loading to the hoisting apparatus is released or a visual or audible alarm signal is generated when the former signal exceeds the latter safety limit signal. In the load limit control disclosed in the U.S. Pat. No. 3,489,294, the load actually applied to the hoisting apparatus is detected by detecting load applied to a hydraulic cylinder unit for luffing a boom of the hoisting apparatus carrying the load through fluid pressure for the cylinder unit. Such detection of load applied to a hydraulic cylinder unit by means of detection of fluid pressure seems reasonable on the face of it, but the detection is not reliable as will be detailed later.
As another method for detecting load applied to a hydraulic cylinder unit in the axial direction of the same according to the prior art is such in that such load is detected by detecting bending moment of a pivot pin for supporting concerned cylinder unit at the bottom of the unit by means of strain gauge or the like fixedly mounted on the pivot pin. That is, such bottom pivot pin is provided when a load carried by the cylinder unit is to be displaced in directions other than the axial direction of the cylinder unit, and a strain gauge or the like is fixedly mounted on the bottom pivot pin so that load in the axial direction of the cylinder unit is detected by measuring the bending moment of the bottom pivot pin which moment is caused by such load. This method does not provide a reliable detection, too, as will be detailed later.